The standard should be set at 20kg for all travelers who are paying to sit on a seat regardless of whether they have booked over 6 months in advance or are on a last minute deal, lets face it the weight of all aircraft are known to the operators either fully loaded or empty, they will also know the spare capacity in which they operate.
the figure should be increased to 25kg for people going for 2 weeks where possible which again would still fall within the aircraft weight limits.
Using the above every person traveling would know the weight allowed and if need be would be charged accordingly for excess baggage by the airlines/carriers.
We always now use the bag type case with wheels and a pull handle because they are lighter, people should maybe look at their cases and what they weigh empty! we have one that is 7kg empty which is quite heavy and only leaves 13kg for baggage.
There needs to be one standard for everyone so that there is no confusion regarding weghts allowed, as for larger people im sure that the things that would be heavier would be their footwear, shirts etc would give very little extra in terms of weight!